Movies for TV - Early Television Foundation

LENSES 135
old one. There is not much point in having projector lenses coated
unless a particularly long throw is involved and the light on the
screen is poor, for in many cases there is too much light
on the
screen in the case of small pictures.
When purchasing a secondhand camera or lens, it is essential
to insist that a picture taken with the lens is available for inspec-
tion and to have the right to try it out on a return basis. This is
even more important if the camera has been used, or is bought
in sandy areas, such as the western deserts or even in Africa.
During the war, many people bought perfect-appearing cameras
for next to nothing from Arabs in Cairo and later found the lens
was useless due to sand blasting by the wind and sand in that area.
For this reason, it is essential to keep a lens capped with a leather
and velvet cap when the camera is not in use. Because the glass
seems so hard many people neglect this precaution with the attitude,
"Oh, it's so 5
hard, nothing can hurt it.' Then a is
picture
made and it is blurred and dark due to sand erosion. If the lens
is coated, then this is doubly important for the coating is com-
paratively soft. Sometimes when a turret is used for mounting
the lenses, one lens which may not be used very much gets badly
worn through lack of attention to this point.
The immediate effect of coating a lens is to increase its / value
by one stop. This sounds very complicated but it is not, for it is
the lens. The
only a measure of the amount of light passed by
foregoing statement is not strictly true but it is commonly stated
that way, and for the moment it can suffice. Later the newest
method of measuring lens transmission by T stops will be mentioned.
The designation / stop was standardized in France early in the
twentieth century to calibrate the openings of the camera lens.
There are two methods of expressing this factor, angular aperture
and aperture ratio. The latter is always used for photography and
the former more for optical work. Angular aperture is given as a
measure of the angle between two lines from the focus of the lens
to two points at the end of the lens ;
this is more difficult to apply
and understand for photographic work. The aperture ratio is just
what it says ; the ratio between the diameter and the focal length ;